The present invention relates to a toner concentration control apparatus for an electrophotographic copying machine, electrostatic recording apparatus or the like, for controlling the concentration of toner in the supply of a two-component type developer comprising at least toner and carrier, by detecting the toner concentration of the developer, and more particularly to a toner concentration control apparatus of the type described capable of detecting abnormalities in the operation of the toner concentration control apparatus, such as stopping of the flow of the developer or clogging of the same in the toner concentration control apparatus.
In order to produce satisfactory copies by an electrophotographic copying machine employing the two-component type developer, it is necessary that the proper ratio of toner to carrier be maintained. However, each time a copy is made, some toner is used for imaging and thus depleted from the supply, and the images produced on subsequent copies will be fainter (at first not noticeably so, but increasingly as more copies are made) unless quantities of toner are added to the developer to replace that which has been used.
In order to eliminate this shortcoming, the development apparatuses of conventional electrophotographic copying machines can be provided with toner concentration control means which operate toner replenishing devices for adding toner to the developer when necessary. Generally, a toner concentration control means comprises a toner mixing-ratio detection means for detecting the ratio of toner to carrier in the developer, and a toner replenishment decision means, which compares the output signal from the toner mixing-ratio detection means with a reference signal indicating, for example, the desired concentration, and which actuates the toner replenishing device in accordance with the decision of the decision means.
An example of such toner concentration control apparatus is disclosed in Japanese Patent Publication No. 38-17245. In the toner concentration control apparatus disclosed, light from a light source is projected onto the developer held in the reservoir, and the light reflected from the developer is detected by a photosensor element. Thereafter, in accordance with the intensity of the reflected light, toner is replenished. This toner concentration control apparatus has proved to be entirely unsatisfactory, since, if the intensity of light from the light source varies due to some variation in the power or for other reasons, the output of the light-receiving photosensor element will be affected. Further, if the photosensor element changes in characteristics, for example, due to age or change in the ambient temperature, its output may also vary. Therefore, the change in the concentration of toner in the developer cannot be detected accurately by the toner concentration control apparatus disclosed in Japanese Patent Publication No. 38-17245.
Furthermore, in this type of toner concentration control apparatus, smearing of the photosensor element with the developer, which is apt to be scattered out of the developer reservoir, is a serious problem causing inaccurate measurement of the toner concentration. Nothing is mentioned as a countermeasure for eliminating such drawbacks and no apparatus is provided for detecting abnormalities in the operation of the toner concentration control apparatus in Japanese Patent Publication No. 38-17245.
Another example of a toner concentration control apparatus, which is an improvement on the above-mentioned Japanese Patent Publication No. 38-17245 to some extent, is disclosed in United States Patent Publication No. 3,756,192. In this apparatus, a chopper wheel, which serves as a calibrated reflector, having a value of reflectance on its blades equal to that of developer of the desired toner concentration, is rotated in front of a single light source. Light is thus alternately reflected from the developer containing toner and from the calibrated reflector, to a photosensor. When an imbalance of toner in the developer results in a difference in reflected light intensity between the developer and reflector, the difference is converted into an electrical signal having an A.C. component. A phase-sensing circuit detects the position of the peak of the A.C. signal component with reference to the position of the calibrated reflector and provides a control signal to a toner replenishing device. This toner concentration control apparatus has an advantage over the aforementioned prior-art toner concentration control apparatus in that utilization of a single light source and a calibrated reflector for reference compensates for variations in output of the light source. However, this toner concentration control apparatus requires a device for rotating the chopper wheel, i.e., the calibrated reflector, which makes the toner concentration control apparatus complex in mechanism, over-sized and expensive. Furthermore, since it contains movable portions, those movable portions may be abraded while in use over an extended period of time, with the result that the reliability of the apparatus may be lowered.
Furthermore, in this toner concentration control apparatus disclosed in United States Patent Publication No. 3,756,192, the calibrated reflector and the photosensor are susceptible to smearing by the airborne toner particles, since they are not protected from the toner particles and an apparatus for detecting the abnormality of the toner concentration control apparatus is not provided.
A further toner concentration control apparatus of the type which senses toner concentration by sensing variations in reflectivity of the developer is disclosed in United States Patent Publication No. 3,830,401. In this toner concentration control apparatus, pulses of radiation (i.e., light), periodically produced by a radiation source at a selected frequency, are directed to the developer mixture, and the reflectance thereof is monitored by a photoelectric transducer which produces a first output signal representative of the intensity of such reflectance. A second photoelectric transducer illuminated directly through the airborne toner cloud in the apparatus by the radiation source produces a second output signal representative of the intensity of the radiation emanating from the source as modulated by the surrounding environment. The toner concentration is detected by comparing the first output with the second output and, in accordance with the result of the comparison, toner is replenished to the developer when necessary, whereby the concentration of toner in the developer is controlled. In this apparatus, variations in output of the radiation source can be compensated for, since the radiation source is shared by the two photoelectric transducers. However, this apparatus requires a pulse generator for energizing the radiation source to produce pulses of radiation at a selected frequency, which is very expensive.
Furthermore, in this toner concentration control apparatus, the radiation source and the two photoelectric transducers are faced with the airborne toner particles circulating within the apparatus. Therefore, it has the short-coming that uncompensated for error will be caused with respect to the radiation source and the two photoelectric transducers if they are smeared differently with the airborne toner particles. Further, an apparatus for detecting the abnormalities in the operation of the toner concentration control apparatus is not provided.
In order to eliminate the above-mentioned short-comings of the prior-art apparatuses, toner concentration control apparatus is proposed, in which light from a single light source is simultaneously detected by two photosensors, the light striking one of the photoconductors directly (or indirectly from a reference reflection and transmission means) and the other after being reflected from developer comprising toner and carrier particles as the developer passes a viewing window in a chute provided for the transport thereof within the development apparatus of a copying machine. By comparing the two outputs of the two photosensors, toner is replenished to the developer supply. The light source and the two photosensors are supported by a rigid support means fixed to the chute and which forms a multichannel light path for the light source therein, and the light path, the light source, the photosensors and the external surface of the viewing window are all sealed from the surrounding environment by the support means. Furthemore, because the two photosensors are substantially identical in characteristics including response to ambient conditions and deterioration thereof with time, the accuracy of the toner concentration determination can be maintained with long-term stability.
In the toner concentration control apparatus of the above-mentioned type, some foreign materials are apt to enter the chute, coagulating the developer, and the chute will be easily clogged with the coagulated developer, causing inaccurate reading of the toner concentration. More specifically, if the chute is clogged with the developer when the toner concentration decreases and replenishment of toner is required, the toner concentration will continuously add toner to the developer supply beyond the desired level, based on the reading of the low toner concentration. On the other hand, if the chute is clogged with the developer when the toner concentration is sufficiently high, no toner will be replenished thereafter even if the toner concentration decreases, and the images produced on subsequent copies will be fainter, although the toner concentration control apparatus indicates that the toner concentration is sufficiently highly.
In order to prevent such improper replenishment of toner, in U.S. Patent Publication No. 4,032,227, there is disclosed a toner concentration control apparatus in which toner is added to the developer supply at predetermined intervals and the frequency of the addition of toner is checked, whereby it is prevented to add excess toner to the developer supply. However, this toner concentration control apparatus requires a counter circuit and other devices for performing the periodical addition of toner to the developer supply and checking the frequency thereof, which makes the apparatus expensive. Further, it has a shortcoming that even if the number of times of replenishing toner to the developer supply is preset, particularly for judging the abnormality of the operation of the apparatus, that preset number may vary, depending upon how often the copying machine is used, and therefore it is not always convenient for practical use.
Furthermore, if the above-mentioned toner concentration control apparatus is applied to a toner concentration control apparatus of the type using a chute for allowing the developer to pass therethrough, and the chute is clogged with the developer or for some other reason, the control apparatus cannot detect such abnormalities immediately and accordingly the copying machine cannot be stopped immediately.